(1) Field of the Invention
The present invention relates to a cathode ray tube (CRT) apparatus for use in television sets and computer displays. More particularly, the present invention relates to a CRT apparatus having a velocity modulation coil.
(2) Description of the Related Art
Edge enhancement is one scheme for realizing high image quality on television. To carry out edge enhancement processing, a television set is provided with a velocity modulation coil arranged at or near a neck portion of a CRT. The velocity modulation coil generates a magnetic field in a vertical direction to modulate the horizontal scanning velocity of an electron beam, thereby emphasizing the appearance of edges in images (See, for example, Examined Japanese Utility Model Application Publication No. S57-45650).
With the recent trend toward color CRT apparatuses having a larger screen, higher luminance, and a flat front panel, the spot diameter of an electron beam on a phosphor screen is larger and the anode voltage is higher. Such color CRT apparatuses require a stronger magnetic field for performing edge enhancement.
To meet the above need, there is suggested a color CRT apparatus capable of increasing the magnetic field that affects the electron beam, without increasing the amount of electric current flowing through the velocity modulation coil or the number of turns of the velocity modulation coil (See, for example, Unexamined Japanese Patent Application Publication No. 06-283113).
In a color CRT apparatus disclosed in the 06-283113 publication, a fifth grid (G5 electrode) of an electron gun that is housed within a neck portion of a CRT has holes for respective electron beams R, G, and B to pass through, and a magnetic member is arranged above and under the holes. In addition, a velocity modulation coil is arranged along an outer circumference of the neck portion at a position corresponding to the G5 electrode. With this structure, magnetic flux generated by the velocity modulation coil is concentrated by the action of the magnetic member to an area through which the electron beams pass. This leads to improve the strength of the magnetic field which contributes to scanning velocity modulation of electron beams.
However, the color CRT apparatus of the 06-283113 publication is insufficient to achieve enough effect. With the disclosed structure, the magnetic field generated inside the G5 electrode (the electron beam passing area) is inevitably weak due to the eddy current loss occurred in the electrode (G5 electrode) made of metal. The magnetic member does strengthen this weak magnetic field but not to a sufficient level. That is to say, the color CRT apparatus disclosed in the 06-283113 publication fails to improve the velocity modulation sensitivity (the amount of modulation in the electron beam velocity relative to input current to the velocity modulation coil) as much as desired. Furthermore, there is another problem. The magnetic member and the G5 electrode are connected together by welding. Naturally, welding of such small components requires a number of manufacturing steps and high manufacturing cost.